1. FIELD OF THE INVENTION
This invention relates to a plasma etching apparatus and more particularly to improvements in uniformity of etching.
2. DESCRIPTION OF THE RELATED ART
FIG. 5 shows a conventional plasma etching apparatus of a parallel plate electrode type. Within a cylindrical chamber 1 is arranged an upper electrode 2 opposing a lower electrode 3. The upper electrode 2 is grounded and the lower electrode 3 is connected to a high-frequency power source 7 disposed outside of the chamber 1. The lower electrode 3 also serves as a sample table, upon which samples 4 to be etched, such as a semiconductor substrate, are placed. The chamber 1 is provided with a gas inlet 5 and a gas outlet 6. Permanent magnets 8 are arranged in the periphery of the chamber 1 encircling it. As shown in FIG. 6, the permanent magnets 8 are so disposed along the periphery of the chamber 1 that the N and the S poles thereof are alternatingly arranged. With this structure, lines of magnetic force 9 parallel to the surface of the sample 4 are present in the chamber 1.
In operation, reaction gases, such as for example, CF4 and CCI4, are fed through the gas inlet 5 and at the same time the gases are discharged through the outlet 6 to constantly maintain gas pressure in the chamber 1. Under this condition, a high-frequency voltage is applied across the upper and lower electrodes 2, 3 in order to generate a gas plasma within the chamber 1. The gas plasma is enclosed in the magnetic field generated by the permanent magnets 8 so that the surface of the sample 4 placed on the lower electrode 3 is etched by exposure to the gas plasma.
However, as is clear from FIG. 6 since the lines of magnetic force produced by a plurality of permanent magnets 8 for enclosing the plasma go in and out among the permanent magnets 8, the magnetic field intensity of the permanent magnets is unequally distributed along the internal circumference of the chamber 1. Further, because these lines 9 are parallel to the surface of the sample 4, the lines 9 tend to cover the area close to the sample 4. Therefore, the density of the ions and the radicals in the gas plasma also becomes unequal, causing a lower etching rate and non-uniformity of etching. This is a problem with a conventional plasma etching apparatus.